9 The city which has given us birth according to the flesh still abides, God be thanked. O that it may receive a spiritual birth, and together with us pass over unto eternity! If the city which has given us birth according to the flesh abide not, yet that which has given us birth according to the Spirit abides for ever. “The Lord does build up Jerusalem.” Has He by sleeping brought His building to ruin, or by not keeping it, let the enemy into it? “Except the Lord keep the city, he that keeps it wakes but in vain.” And what “city”?
“He that keeps Israel shall neither slumber nor sleep.” What is Israel, but the seed of Abraham? What the seed of Abraham, but Christ? “And to your seed,” he says, “which is Christ.” And to us what says he? “But you are Christ's, therefore Abraham's seed, heirs according to the promise.” “In your seed,” says He, “shall all nations be blessed.” The holy city, the faithful city, the city on earth a sojourner, has its foundation in heaven. O faithful one, do not corrupt your hope, do not lose your charity, “gird up your loins,” light, and hold out your lamps before you; “wait for the Lord, when He will return from the wedding.” Why are you alarmed, because the kingdoms of the earth are perishing?
Therefore has a heavenly kingdom been promised you, that you might not perish with the kingdoms of the earth. For it was foretold, foretold distinctly, that they should perish. For we cannot deny that it was foretold. Your Lord for whom you are waiting, has told you, “Nation shall rise up against nation, and kingdom against kingdom.” The kingdoms of the earth have their changes; He will come of whom it is said, “and of His kingdom there shall be no end.”
Source: Sermons on Selected Lessons of the New Testament (New Advent)